Molten salts, such as a potassium nitrate (KNO3) and sodium nitrate (NaNO3) mixture, are used as a thermal transfer medium in solar power plants. Typically, reflectors are used to concentrate solar energy at a receiver. The receiver operates as a heat exchange device to raise the temperature of the molten salt as the molten salt is pumped through the receiver. The outside surface of the receiver can reach temperatures that exceed 1200° F. (650° C.). The molten salt is then transferred to a thermal storage tank and then to a heat exchanger to generate steam.
Molten salts are chosen for their heat transfer characteristics and handling requirements. Mixtures of KNO3 and NaNO3 salts are in the liquid phase while in the operational temperature range of a solar receiver facility. The operational temperature range for a solar receiver facility is typically 550° F. to 1050° F. (290° C. to 560° C.). For power generation, the molten salts are necessarily heated in the solar energy receiver and cooled in the heat exchanger.
The heat exchanger typically transfers this thermal energy to a working fluid such as water. The heated water is converted to steam in the heat exchanger which can be used to power a steam turbine-generator to produce electricity.
The piping and equipment used to transport the molten salts experience thermal expansion and contraction as the system is started, operated and shut down. As with most land-based solar power applications, this cycle is repeated daily as the sun rises to heat the receiver. Most of the piping and equipment of a solar facility are allowed to cool to ambient temperatures at night. Thermal expansion of equipment and piping will lead to induced stresses as pipe lengths increase between fixed points. With a receiver located atop a tower several hundred feet in height, several inches of axial thermal expansion must be accommodated. Expansion loops, such as a series of pipe sections connected by 90° ells or a piping spiral, can be installed in piping to relieve some of these induced stresses, but require more piping which increases material costs and thermal losses. Additionally, expansion loops require more space, maintenance and installation costs and time.
Another issue that arises when using molten salt in a solar power application is leak prevention. Both KNO3 and NaNO3 are severe oxidizers and contact with fuels at elevated temperatures can cause combustion. Penetrations into the molten salt equipment, such as valve stems, are potential areas for leaks. Periodic inspection and maintenance are required to ensure that any leaks are minimized.
Molten salt is typically required to be at temperatures in excess of 500° F. (260° C.) to remain in the liquid phase since its freezing temperature is approximately 430° F. (220° C.). Equipment used to transfer molten salt is maintained above this minimum temperature to ensure that the molten salt does not freeze. Electrical heat tracing is typically used to maintain this minimum temperature. Freezing of the salt in equipment can cause reduced flow up to a total flow stoppage. In the event of a freeze out of salt in a length of piping or equipment, special care must be exercised when thawing the salt. Upon thaw, salt expands and can damage pipe and equipment if a free surface is not available. If the molten salt cannot be fully drained from pipes or equipment, solidification will occur as the salt is cooled. These solid portions of salt can restrict flow or damage equipment upon restart.
Conventional valve stem seals for molten salt applications use dynamic friction seals. Most valve stem seals experience a high short term failure rate at an operating temperature of 1050° F. (560° C.). In previous solar molten salt facilities, the valve stem and bonnet were lengthened in order to separate the valve stem seal from the flow of molten salt. This modification provided a lower operating temperature for the valve stem seals and resulted in a longer service life. With longer valve stems, valve failures such as plastic stem twisting and stem buckling become more likely. Additionally, valve locations are limited when greater clearance is needed for the lengthened valve stem/bonnet.
What is needed is a containment device for molten salt that will accommodate axial expansion, withstand thousands of cycles, reduce the potential for leaks, and allow maximum drainage while providing a long operating life at elevated temperatures.